The invention relates to a balance with a weighing pan surrounded on all sides by a weighing compartment that is delimited by a draft shield. At least one vertical wall of the draft shield is formed by a component of a stationary part of the balance. The balance has at least one thermoelectric module arranged outside of the weighing compartment and thermally connected to the stationary portion of the balance.
Balances of the kind used primarily in laboratories where stringent accuracy requirements must be met, for example analytical balances, often have the problem that the heat dissipation of the weighing-cell electronics and indicator electronics will warm up the air inside the weighing compartment while the draft shield is closed. When the draft shield is subsequently opened, for example to place an object on the weighing pan, the interaction with the outside air causes a relatively strong convective air current. Even after closing the draft shield, a convective air circulation will continue for some time inside the weighing compartment, causing the weighing result to remain unstable for a considerable length of time, in some cases until the convective circulation has returned to a stationary state. However, in most cases the air strata in the weighing compartment remain unstable, albeit only to a slight degree. As a rule, this will cause a continuous air circulation in the weighing compartment, so that the weighing result is subject to continuous, albeit slight fluctuations.
The German patent application DE 100 31 415 A1 discloses an analytical balance with a balance housing consisting of a housing base and a housing top, and with a balance pan surrounded on all sides by a draft shield that encloses a weighing compartment. The balance is equipped with a means for producing an upward-directed narrow stream of air in the weighing compartment or in a space connected to the weighing compartment. The purpose of the air stream is to create in the weighing compartment a temperature distribution that has a favorable effect on the weighing performance. According to the proposed concept, the favorable temperature distribution is achieved through a controlled small amount of stationary air circulation in the weighing compartment. The air stream can be generated in different ways according to different embodiments of the balance, for example by means of a heat source of compact dimensions that is attached to the upper half of the rear wall of the weighing compartment, or by means of an air duct running between an opening in the lower half and an opening in the upper half of the weighing compartment and containing an air-stream generating element. The air-stream generating element can be a fan or a heat source.
All of the embodiments described in DE 100 31 415 A1 have in common that they use an element which introduces an additional amount of heat into the balance which already carries the thermal load caused by intrinsic power dissipation. Consequently, the weighing compartment will be at a temperature that is predominantly above the ambient temperature, in some cases even by several degrees Celsius, and thus this concept fails to prevent the aforementioned air draft that occurs when the draft shield is opened. There is further the risk that any slight air circulation at or near the weighing pan will affect the weighing result, even if the circulation is intentionally created.
The Japanese patent JP 2 586 115 B discloses a balance in which the weighing compartment is temperature-controlled in such a way that the inside of the weighing compartment is at about the same temperature as the ambient air, thereby avoiding the problem of convective air currents when the draft shield is opened. This is accomplished by a thermoelectric module that is arranged outside the weighing compartment, preferably in the upper part of the balance housing. The thermoelectric module is in thermal contact with a cooling plate that runs along the wall or actually forms the wall separating the weighing compartment from the balance housing that contains the weighing-cell electronics. The purpose of the cooling plate is to keep the heat generated by the electronic components away from the weighing compartment, so that the air inside the weighing compartment will always be at the same temperature as the outside air. Thus, no temperature-induced convection occurs between the weighing compartment and surrounding space even when the draft shield is open. Likewise, no thermal effect on the weighing result is caused by bringing the weighing object from the outside into the weighing compartment, because the weighing container, the weighing sample itself, as well as the outside air enclosed in the weighing container are already at the same temperature as the inside of the weighing compartment.
However, based on the drawings of the Japanese patent JP 2 586 115 B, it is felt that the proposed arrangements of the thermoelectric module present problems of their own. The placement of the thermoelectric module, preferably in the upper part of the balance housing, may be adequate for the purpose of removing heat from a cooling plate that serves as a thermal shield for the electronic module, so that the temperature of the weighing compartment is approximately matched to the ambient temperature. However, the proposed solution is fraught with the risk of an unstable stratification of the air in the weighing compartment, as it tends to create a temperature distribution with colder air on top and warm air in the lower part of the weighing compartment. This has the inevitable consequence of a convective air circulation inside the weighing compartment, which in highly sensitive balances will manifest itself through fluctuations in the weighing result.